The decay of the Higgs boson into two photons was among the main discovery channels at the Large Hadron Collider (LHC) [1]}, [2]} and plays a crucial role in precision studies of its properties.
Among these are measurements of its mass, see e.g. [3]}, as well as the study of the interference between the di-photon signal with the continuum background, which leads to a shift in the di-photon mass spectrum
and in production rates, thus allowing for studies of the total Higgs boson width [4]}, [5]}, [6]}, [7]}, [8]}, [9]}.
In addition, future colliders will allow the measurement of ratios of branching fractions at the sub-percent level (see e.g. [10]}), thus demanding
theoretical predictions of partial decay widths at the same level of precision.
